In a numerical control machine tool that conducts machining accurately and precisely as instructed by an NC machining program, it is important, in terms of quality control and productivity, how to create the machining program easily and efficiently that yields a machining result as intended by a designer who designs an object to be machined.
Along with the recent enhancement of an NC program creation assisting function of a program creation apparatus, it has become possible for an operator to create an NC machining program easily by setting coordinates of machining subjects while referring to a production drawing. An apparatus has appeared that can create an NC machining program by directly importing computer aided design (CAD) data modeled by a designer using a CAD system.
For machining a region of such as fit (mating) and a dimensional tolerance which is related to the function or performance of an object to be machined as a product, it is necessary to have an NC machining program reflect a machining target dimension considering the fit or the dimensional tolerance. Especially, a machining target dimension may be a value different from a nominal dimension in the case of a fit or a dimensional tolerance in which permissible upper and lower dimensional deviations (dimensional upper limit and dimensional lower limit) are asymmetric being one-sided to either one (dimensional upper limit or dimensional lower limit).
When an NC machining program is directly programmed (modified) while referring to a production drawing, an operator calculates a machining target dimension manually or by using a calculator, and a coordinate compensated based on the calculation result is input to the NC machining program. In this method, miscalculation or input error is likely to occur and the created NC machining program becomes unreliable because modification of the NC machining program is complicated.
To solve this problem, a method has been proposed that enables to directly write nominal dimensions and tolerance information (dimensional tolerances or the like) in the NC machining program and making a numerical control apparatus perform machining process based on the written information.
For example, a numerical control apparatus described in Patent Document 1 stores therein dimensional deviation data based on the nominal dimension and tolerance zone class of a fit. The apparatus calculates the central position of a tolerance zone based on a command in the machining program that describes the nominal dimension and tolerance zone class of a fit of a machining object as well as the stored dimensional deviation data corresponding to the command, and then performs axis control using the calculated central position as a move command of the controlled axis. Therefore, a programmer can directly program the nominal dimension and tolerance zone class of a region of a fit written on a machining drawing.
An automatic fit compensator of a numerical control apparatus described in Patent Document 2 detects a fit symbol sandwiched by predetermined bound symbols from an NC command program, and calculates a cut dimension based on the dimension corresponding to the fit symbol and the dimension in the NC command program.
In a machining control method described in Patent Document 3, when a dimensional deviation is indicated with a specific numeric value in a dimensional deviation description area in a machining program, a machining target dimension is determined based on the numeric value as indicated, and when a dimensional deviation is indicated with a fit code, a dimensional deviation table is searched, then a machining target dimension is determined.    Patent Document 1: Japanese Patent Application Laid-open No. 4-245305    Patent Document 2: Japanese Patent Application Laid-open No. 61-15204    Patent Document 3: Japanese Patent Application Laid-open No. 60-201860